PdfScrollablePageView: improve positional navigation

Similar to f467edc9

:
NavigationStack is kept up-to-date when the scroll position changes, and
can go back to a previous position; and links can in theory jump to
specific positions and zoom levels, scrolling such that a specific x
coordinate is visible.

Change-Id: I2add617914d89b0dc5389e7c3d12d11580a1f82f
Reviewed-by: Shawn Rutledge <shawn.rutledge@qt.io>